Method of agglomerating and sintering granular lead sulphide



United States Patent I METHOD OF AGGLOMERATING AND SINTERING GRANULARLEAD SULPHIDE Bjiirn Erik Anders Berglund, Bromma, and Karl GiiranGtirling, Nils Berti] Lindvall, and Sven Johan Wallden, Skelleftchamn,Sweden, assignors to Bclidens Grnvaktieboiag, Skelleftehamn, Sweden, ajoint-stock company limited of Sweden No Drawing. Application February6, 1956, Serial No. 563,452 Claims priorit application Sweden June 20,1951 11 Claims. (Cl. 75-5) This is a continuation-in-part of ourapplication Serial No. 293,213 of June 12, 1952 now abandoned.

The present invention relates to a process of sintering andagglomerating high-grade concentrates of lead sulphide minerals,containing the lead substantially combined as sulphide.

The raw material generally used for the production of lead in the leadplants, is galena. Ore in the form of large pieces is seldom used but isinstead crushed and concentrated at the mine. By the modern flotationconcentration methods now in use, it is possible to exceed the contentof 50 60% lead and sometimes reach a value of about 80%. At this valuethe product comprises a substantially pure galena, the composition ofwhich is 13.4% sulphur and 86.6% lead. A great part of the ore, which isnow treated by the lead plants, is highly concentrated product with leadcontents exceeding about 55%. In order to produce lead from theconcentrates it is usually necessary to convert the fine granularmaterials into a sintered or agglomerated form, viz. subject thematerials to a heat treatment in the presence of oxygen on a grate,while simultaneously removing a substantial portion of the sulphurcontained in the product in the form of S02.

In Trans. A. I. M. E., vol. 159 (1944) p. 81, Some experiments onsintering lead sulphate products it is stated that a material subjectedto sintering, having the usual gangue components and containing morethan about 50% lead, tends to fuse during the sintering operation withthe result that the grates and the charge are clogged. The result ofthis is that the air does not come into sufliciently good contact withthe sulphides at the roasting temperatures so that the removal ofsulphur is thereby small. For the lead concentrates mentioned above,containing about 55-80% Pb, it would be necessary to add diluentsthereto and thus decrease the content of lead to about 50% or less.Although in the article this method is stated to result in economicaldisadvantages and is therefore not an ideal one, the method is used allover the world.

The most important disadvantages are:

1. The expense of providing barren diluents. 2. The diluents lower theblast furnace capacity. 3. Increased slag losses.

Moreover the heat consumption for smelting increases.

In the article it is also stated that experiments conducted in order todesulphun'ze high-grade lead concentrates by means of methods difierentfromsintering do not appear to be successful. Thus, it is stated thatflash roasting converts most of the lead sulphide into lead to thediificulties in effecting sintering. These difliiculties tend to risevery rapidly above a lead content of 55%.

The reason for this is that when sintering by the present sinteringmethods, i. e., in which the charge is prepared by mixing the differentcomponents to a fine granular mass having a suitable content of waterand with lead sulphide material charges having contents of lead ex;ceeding the value of about 55 large amounts of lowmelting phases areformed which render it difiicult to perform the sintering operationtechnically. As little as about 50% Pb in the charge will causeconsiderable difliculties in the sintering operation. The most importantdifiiculty is that low-melting products deriving from easily fusiblephases will flow down to the lower parts of the charge and between thebars of the grate and will also adhere to said bars, whereby the flow ofthe air through the charge subjected to the sintering is obstructed.This results in additional work, i. e. to clean the bars and collect themolten material flowing down through the spaces between the bars. Asmentioned above it will also result in unsatisfactory air passage sothat an insufiiciently desulphurized agglomerate is obtained.

The reason for the formation of such great amounts of lead whensintering high-grade lead concentrates will be clear when studying theequilibrium investigations of the system ib-S-O, such as disclosed bySchenk, Metall und Erz 1926, number 15, pages 407420: Experimentelle undtheoretische Untersuchungen uber die Rostgleichgcwichte beim Blei. Aconstitutional diagram of the Pb-SO-compounds in which the temperatureand SOz-pressure are varied shows that it is necessary to reach thetemperature of 300 C. or more in order to eliect the utmost possibledesulphurization. To avoid a precipitation of lead at such hightemperature the S02- pressure should be higher than about mm. Hg. Insintering it is possible to recirculate the waste gas to increase theSOz-pressure. However, by such a step it is not possible to obtain thesaid necessary minimum pressure.

Heretofore it has not been technically possible to perform a sinteringof very lead-rich charges. It has been necessary to mix suchconcentrates with lead-poor or inert materials so that the lead contentsof the sinter charge are appreciably below 50%. The reason is that thelow-melting Pb-O-S-phases, which are formed, clog the charge and thegrates and make the gas-passage more diiiicult. Owing to the high localtemperature of the combustion zone and the low SOz-pressure, lead inconsiderable amounts is formed. This metal may trickle through thecharge and solidify in contact with the grates or may drip down to thewind boxes. Such diificulties make the process impossible. The mainobject ofthe sintering is to bring down the sulphur content of theagglomerate to proportions suitable for the reactions between PbS andPbSOt+PbO, with respect to the sidereactions. Preferably the content ofthe sulphur combined as sulphate should be as low as possible. It hasbeen established that the reactions between PbS and oxygen cannot beperformed in solid state with desirable reaction speed and desirableresult. At temperatures below 790 C. basic lead sulphates are the finalproducts (see diagram by Schenk). If the temperatureis increased abovethis point, a range is reached in which low-melting eutectics exist.Consequently it is necessary to learn to control these difiiculties. Attemperatures around or above 800 C. the roast reactions lead to :PbO andPb (see Schenk). First at temperatures around ESQ-900 C. the speed ofthe reactions resulting in Pb is appreciable. Therefore it is notadvisable to heat the reacting charge to temperatures above 850. C.forextendedperiods of,time. The controlling of the temperature isfacilitated by the partial melting, starting at about 800 r C. Thereforeit'is of deciding importance to find a method to control the inevitablemelting process. Sintering is industrially performed today in such a waythat the lead compounds are diluted with slag components of high meltingpoint, said components forming a highmelting skeleton, which preventslow-melting phases to run together. Since it is intended to prepare acharge having a lead content of appreciably more than 50% and it is notdesirable to dilute the lead-rich charge with material poor in lead,there is no other diluent available than return sinter and dust from theprocess. In finely divided state these materials will react with thelead sulphide. Coarser return sinter particles, however, will only reacton the surface and the core remains intact. Ifit were possible to applythe concentrate and other fine-grained materials on the surface of suchcoarse particles, the sintering reactions should take place in a thinsurface layer and it might be possible to solidify the partially moltenreaction products by air-cooling before they drip from the core.

The object of the present invention is to provide a process by which itisv possible to remove sulphur from the high grade lead concentratescontaining at least about 55% lead to a value preferably not exceedingabout A second object of the invention is to provide a desulphurizationprocess, Without easily fusible products including elementary lead beingformed and flowing downwardly through the charge or adhering to the barsof the grate, and thereby diminishing the supply of air.

In addition to lead, the concentrates that are to be used according tothe present invention, contain sulphur as a second main component. Thesulphides act as a source of fuel in the sintering operation. In theconcentrates that may be present a gangue consisting of quartz, silicicacid, silicates, aluminium oxide, oxides of alkaline metals and alkalinerare earth metals, metal sulphides, such as zinc sulphide, ironsulphide, pyrite, etc., may be present. Examples of concentrates, whichmay be used according to the invention, are revealed in the examplesstated below. The concentrates that are to be used according to thepresent invention are in a very fine granular form, by which is meantthat the particle size is generally below about 0.1 mm. More preferablythe particle size distribution of the concentrates is such that at least90% of the material will pass a screen of 240 mesh. For the ball rollingstep it is of importance that the lead sulphide material is in a finegranular state. Thus the ball rolling is favoured by employing as fine agranular material as possible.

In its broadest form the invention resides in that particles of inertlead oxide containing material, preferably derived from a precedingsintering or agglomeration step and showing an average particle size ofsubstantially above about 5 mm. but not more than about 30 mm.particularly not more than about mm. are provided with a coating of thelead sulphide material, viz. the high-concentrated fine granular leadsulphide material, so as to form a ball structure charge. The coating ofthe particles or pieces is effected in such a way that the balls willconsist of an inner core of lead oxide containing material, particularlydesulphurized, agglomerated material, and a shell of said fine granularlead sulphide materialin a layer of thickness of preferably about 1-3mm. surrounding said core. Said layer should be thin so as to allowoxidizing gas to come into excellent contact with said sulphide materialof the shell.

The use of such thin shells of the lead sulphide material to be treatedenables a rapid desulphurization of said shells. By employing suchshells there will not be liberation of sufiicient heat to bring the coreof the balls to fusing temperatures. Since the core does not contain anysubstantial amount of sulphides it is not necessary to heat said core toa desulphurizing temperature.

The SOz-containing gas formed in the sintering reaction h diffusesthrough said shell, causing high local SOz-pressure.

Due to the thin shell the risk that easily fusible phases includinglead, formed within the shell, will flow together and trickle down,clogging the charge is diminished. Owing to the suggested measure thecore of such a ball will. have a. cooling effect on the shell during thereaction between the lead sulphide and the oxygen, whereby the oxidationof the lead sulphide of the shell will be substantially finished beforethe core consisting of oxidic material has become completely heatedthrough and the head part of the core has reached fusing temperature.

In the preparation of the balls, devices well-known in the art aresuitably used, c. g. rotating drums, cylinders, discs, etc. The oxidicmaterial used for the core is placed in said device and the finegranular lead sulphide material is added thereto in portions orcontinuously while simultaneously moistening the mass. The conditionsare controlled so as to form round balls in a manner known to everybodyskilled in the art. In order to cause the lead sulphide material toadhere to the surface of the pieces of lead oxide and to effect the ballrolling it is necessary to add moisture or water in operation. Ofcourse, the amount of water must not exceed that amount at which thefine granular material forms balls per se. The addition of water isabout 5% calculated on the whole charge. The amount of lead sulphidematerial, which is added to the lead oxide containing material, isdependent on the content of lead in the lead sulphide material and onthe gangue therein. For a lead sulphide material, containing about 55%Pb, viz. the lower limit for the material used in the present invention,an amount of oxidic material of about 30% calculated on the whole chargeis used. For increasing the content of lead in the lead sulphidematerial the amount of lead oxide containing material must be increasedin view of the fact that the lead sulphide material containing higherlead content will more readily form low melting phases and precipitatelead during the sintering. For a content of lead in the lead sulphidematerial of about lead it is suitable to use a content of lead oxidematerial of about 60%. Material of higher lead oxide content may also beused, if the impurities are particularly capable of forming low-meltingphases easily. A man skilled in the art is capable of estimating howmuch lead oxide containing material should be used in the charge withinthe stated limits of about 35-60% in order to control the formation ofeasily fusing material and elementary lead being formed in a subsequentsintering operation.

According to a special embodiment of the present invention it hasappeared to be particularly suitable to coat the pieces of lead oxidecontaining material with a thin layer of calcium carbonate or lime rockdust, or a d'lfierent heat consuming material, e. g. other carbonatessuch as magnesium carbonate, or dolomite, hydrates, crystal Watercontaining salts or similar compounds, prior to applying the shell oflead sulphide material. This step is recommended particularly for highlead content in the lead sulphide material, e. g. about 7080% Pb. Theadvantage of using such a heat consuming layer, preferably consisting ofcalcium carbonate, resides in that more concentrate can be applied tothe pieces of lead oxide ma terial without any risk that the formationof low melting phases cannot be controlled. This discovery isparticularly surprising and enables further economical advantages inperforming the process according to the invention.

It is also within the scope of this invention to apply a material,containing lead oxide, e. g. lead oxide dust derived from filterdevices, such as finely dispersed lead oxide and lead sulphates of thekind precipitated by electrostatic filters and fines from crushing thesinter as well as slimes, skimmings, etc.,to particles or pieces of leadoxide materials; Preferably such a layer is applied to the oxidicparticles prior to applying the layer of lime rock dust.

According to the invention the balls consisting of an inner oxidic corehaving a surrounding shell of lead sulphide material and obtained byball rolling in the described manner are spread on a grate of asuit-able sinter apparatus. Such an apparatus comprises an endless beltforming a web composed of a number of grate bars, on which web thematerial to be agglomerated is placed in form of a layer. This layer mayhave a thickness of about -30 cm. preferably not more than aboutSO cm.and not below about 5 cm. After having spread out the material to besintered on the grate the upper surface of the same is ignited, i. e. bymeans of a burner fed with oil, gas or coal powder. A stream of air issuctioned through the charge, causing the high temperature combustionlayer to proceed downwardly against the grates. The combustion gasesformed preheat the underlying charge and the combustion air is preheatedby the already sintered layer, while simultaneously cooling the layer.In the sintering operation it is an object to provide such conditions aswill not cause any harmful fusing. If harmful amounts of low meltingphases are formed on the surface of the balls this will result in theflowing of low melting products downwardly through the charge to adhereto the grate bars. As a result the supply of air for the desulphurizingwill be decreased.

Generally sintering machines of the type Dwight Lloyd are used. Suchmachines are well-known to everyone skilled in this art. They consist ofa number of flat pans having a bar grate at the bottom and which arecombined to a forwardly travelling belt. The charge is fed continuouslyfrom a hopper to the grate, on which the surface is ignited by means ofa flame. One or more wind boxes beneath the pan-s are connected with afan capable of causing a sufiicient stream of air to be suctionedthrough the charge. At the outlet end of the sintering machine thesintered product, if not adhering to the grates, is thrown offautomatically and if desired sprayed with water for cooling and thendelivered to a crusher over a screen, in which the return material isseparated. Another suitable apparatus for sintering is Rolfsenssintering machine according to the U. S. Pat. No. 2,193,698 (1940) and2,493,421 (1950) with the watercooled grates arranged in form of atruncated con-e. Other suitable apparatus are large flat pans, in whichthe charging, combustion and throwing ofr occur discontinuously(Greenawalt etc). According to the invention the prepared char-geconsists of balls of oxidic material surrounded by -a shell of leadsulphide is applied to such a sintering apparatus. Air is then caused topass through the same, after the charge has been ignited by means of aflame. After the ignition the surface of the balls will reach thenecessary reaction temperature of 800 C. and more very rapidly and thereaction will proceed towards the core of the balls while simultaneouslydesulphurizing the outer layers of the shells. Due to the thinness ofthe layers the reaction will proceed very rapidly and will be finishedbefore the core has been heated through. Said core will thus have acooling effect on the shell and will consume a portion of the heat thusformed in the reaction. Moreover, the lead forming reactions will nothave sufiicient time to proceed during the sintering due to the rapidoxidation reaction. Inasmuch as the charge has become porous because ofthe special method of preparation, it is not difiicult to control theamount of air passing therethrough. If there are any signs of theformation of low melting phases the amount of air passing the sintercharge is increased, so that a cooling of the surface layerof the ballsis effected. If, on the other hand, the sintering temperature tends tobecome too low, so that the'desulphurizing effect ceases, the amount ofair passing through the sinter charge on thegrate is decreased. On

investigation of the sinter it has been found that the precipitationoflead and low melti-ngaphases is very low aso sae and is generallybelow about 1% of the sinter. The in:

vention involves rapidly bringing the surface of the balls to thenecessary reaction temperature of at least about 800 C., allowing thereaction to proceed through the thin layers of the balls very rapidlyand cooling the balls very rapidly. Due to the ball structure of thecharge material the oxidizing gas, viz. normally the air, can pass saidcharge very easily at a large velocity and simultaneously imp-art acooling effect to the charge. The spaces between the balls oiferpassages, through which the air can pass easily without too much powerbeing consumed. The layer of lead sulphide on the surface of the leadoxide material is moreover relatively thin so that the penetration ofoxygen into the interior of the shell and the diffusion of the sulphurdioxide formed will be improved. It is substantially the above mentionedfactors thatwill be responsible for the particular advantages incarrying out the sintering operation.

According to the invention a portion of the sintered product is recycledand used for subsequent sintering operations. According to a suitableembodiment of the present invention the sintering is carried out bysuctioning air through the charge. In this process it is suitable toarrange a number of wind boxes beneath the sintering belt and controlthe pressure of said wind boxes so as to maintain the suitable stream ofair according to the temperature of the charge and according to signs ofbeginning formation of low melting phases. In the present process it isgenerally suitable to increase the flow of gas through the chargetowards the end of the sintering.

According to the present invention it is possible to produce a sinterproduct, containing about 70-80% Pb and about 2-5% S. This valueincludes the contents of sulphur combined as sulphate as well as anamount of sulphur combined as sulphide. Such a material can be smeltedaccording to conventional methods in a shaft furnace.

As stated above the invention also relates to a new method for preparinglead from high grade lead concentrates. According to this feature of theinvention a sinter obtained according to the method described above issmelted. When smelting said sinter the proportion of the lead sulphideto the oxygen containing lead compounds should substantially correspondto the stoichiometric proportions for reacting to metallic lead.

Lead forming reactions, which occur in such a smelting procedure, can bestated by the formulas PbS+2PbO=3Pb+SOz-5S.6 kilogram calories 1)PbS+PbSO4=-2Pb+2SOz -94.4 kilogram calories In case the material to besmelted consists of the sinter in accordance with the present inventionthe smelting reaction will predominently occur-in accordance with theFormula 1 above. Said formulas show that a reaction according to (2)requires about 2.5 times as much energy per unit of lead as the reactionaccording to (1) and that it involves three times as great an amount ofwaste gases per unit of lead. By smelting the sinter obtained inaccordance with the present invention i. a. the following advantages aretherefore obtained in comparison with the conventional blast furnaceprocess.

1. The energy consumption per unit of lead for smelting is considerablylower and the smelting capacity increases.

2. The amounts of waste gases produced are lower, which involves lowerlosses of dust.

3. The lead losses with the smelting furnace slag are decreased.

The following advantages are obtained in comparison "7 carryout .aconverter blowing after the smelting operation. i

- 3. The circulating dust quantity can be kept very low.

4. The amount of gas from the smelting furnace decreases.

The following advantages are obtained in comparison with the Newmanhearth smelting process.

1. Higher direct lead output.

2. No slag treatment necessary.

3. Lower dust amount.

In certain cases it canbe suitable to reduce the sulphur contents of thesinter to a lower value than required for carrying 'out the leadformation according to the reactions stated aboveinithe Formulas l and2. A sinter having'such a low sulphur content can be obtained accordingto the present invention. The lead oxide excess can be reduced in thefurnace, e. g. by using carbon or lead concentrate.

Any small amounts of sulphur, arsenic, antimony, tin, a. s. 0. presentin the'molten lead discharged from the furnace can easily be removed byblowing in oxygen or oxygen enriched air, or air eventually preheated.When using pure oxygen gas very small waste gas amounts are obtained andsubstantially no dust losses occur. The blowing with oxygen can e. g. becarried out directly in the ladle, into which the lead is dischargedfrom the furnace, or in a well connected to the discharging spout,continuously during the discharging.

The invention will now be described With the'following examples.

' Example 1 A sinter charge having the following composition wasprepared:

Parts Sintered material returned from a previous sintering step andhaving a particle size of about 3-40 mm 45.5 Lead dust 5.4 Lime stonepowder 3.5 Lead concentrate 45.5

This lead concentrate had a particle size less than 0.1 mm. and thefollowing analytic composition: 78.4% lead, 12.6% sulphur, 0.5% zinc,0.5% iron, 5.3% SiOz, 0.4% A1203, 0.2% MgO and 0.2% CaO. The sinteredmaterial recovered from a previous sintering step was moistened in arotating drum with sufiicient water to dampen the surface (approximately4.0 parts), and the lead dust was then added to the rotating drum, toform a basic lead sulphate coating on the sintered particles. The leaddust consists substantially of lead oxide and lead sulphatesubstantially below 200 mesh recovered from an electrostatic filter. Thelime stone powder of air swept quality was then added while adding afurther amount of water by spraying, to form a further coating on theparticles. Finally the lead concentrates were added, forming an exteriorcoating on the moistened balls. The product thus obtained was of ballstructure, and formed a charge particularly permeable to gas. The chargewas thus very suitable forcarrying out the subsequent sintering. For thepurpose of sintering said charge material was fed to a relatively smallDwight Lloyd machine having four wind boxes and an effective suctionarea of one square metre. The grate was first coated with a 30 mm. layerof sintered material in a manner well-known in the art. The total heightof the charge applied to the machine was 250 mm. The vacuumconditionswere controlled in such a way thata'vacuum of 50-80 mm. water gauge wasmaintained in the ignition wind box, No. 1. In the wind boxNo; 2 thevacuum was 150-200 mm. water gauge. In wind box No. 3 the vacuum wasabout 450 mmLwater gauge. In the last wind box No. 4, the vacuum was 400mm. Water gauge. Under these circumstances it was possible to sinter thematerial without any formation of low melting phases which would flowdownwardly, clogging the charge and adhering to the bars.

More air was passed through the sinter cake in the last stage than inearlier stages of the sintering to cool the hottest part of the sintercake next to the grates. At the top temperature of about 800900 C. thereappeared to be some melting of the surface coating, but said melting wasnot so remarkable as to cause any phases to flow down to the bars. Theinner portion of the balls did not have sufi'icient time within which tobe warmed to the fusing temperature The composition of the resultingproduct after sintering showed 5% total sulphur, 1.5% of which waspresent as sulphide, 20-25 tons per day of lead concentrates may PartsSinter 93 Lime (calculated as CaO) 4 Coke breeze 3 240 kg. slag,containing 3% Pb, were obtained per ton of lead.

Example 2 The sintering process of Example I was carried out with I thefollowing charge:

, Percent Returned sinter (particle size 4-12 mm.) 54.5 Lead dust 4.5Lime stone powder 3 Lead concentrates 38 The composition of the leadsulphide material was as follows: 79.1% lead, 12.8% sulphur, 0.6% zinc,0.4% iron, 4.9% SiOz (gangue), 0.3% A1203, 0.2% MgO and 0.1% CaO.

The sintered product analyzed 75% lead and 2.2% total sulphur of which0.9% was present as sulphate. In this case from 18 to 20 tons of leadconcentrate was sintered per day per square meter of effective sinteringarea. As shown in Example 1 above, the lead separation was very small.In Example 2, the particle size of the return sinter was keptconsiderably smaller than the particle size of the product charged inExample 1 above. This increased the surface area of the charge anddecreased the thickness of the sulphide layer on the cores which isbelieved to be responsible for the decrease in total sulphur content.

Having now particularly described and ascertained the nature of our saidinvention and in which manner the same is to be performed, wedeclarethat what we claim is:

1. In a method ofagglomerating and sintering a fine granular leadsulphide material containing at least about 55% lead, whilesimultaneously desulphurizing the material and forming an oxidecontaining product, the improvement which comprises coating particles oflead oxide containing material derived from a sintering operationcarried out previously and having an average particle size within therange of about 5-30 mm. with a thin layer of said lead sulphide materialby ball rolling, said ball rolling being controlled so as to form ballshaving a shell of said lead sulphide material of a thickness within therange of about 1-3 mm. said oxidic material being present in an amountof at least about 30% and not more than about applying the materialprepared by said ball rolling consisting of inner oxidic cores andsurrounding shells of said lead sulphide material on a sintering gratein the form of a charge layer permeable to well gas, igniting saidcharge, causing oxygen containing gas to pass through said charge so asto effect the sintering without allowing easily fusible products to flowdown through the charge and cooling said charge rapidly subsequent tosaid sintermg.

2. A method as claimed in claim 1 characterized by effecting thesintering while controlling the flow of air through the charge in such away that no easily fusible products flow down through the charge.

3. A method as claimed in claim 2 characterized by, at signs ofbeginning flow of easily fusible phases through a portion of the charge,increasing the flow of air through said portion of charge.

4. A method as claimed in claim 2, characterized by decreasing the flowof air through a portion of the charge when the temperature of saidcharge tends to fall below the temperature necessary for effectingdesulphurization.

5. A method as claimed in claim 1, characterized by effecting thesintering while controlling the contents of heat liberating and heatconsuming components of the individual balls.

6. In a method for agglomerating and sintering a fine granular leadsulphide material containing at least about 55% lead, whilesimultaneously desu-lphurizing the material and forming an oxidecontaining product, the improvement which comprises coating particles oflead oxide containing material derived from a sintering operationcarried out previously and having an average particle size within therange of about 5-30 mm. with a thin layer of said lead sulphide materialby ball rolling, said ball rolling being controlled so as to form ballshaving a shell of said lead sulphide material of a thickness within therange of about 1-3 mm., said oxidic material being present in an amountof at least about 30% and not more than about 65 applying the materialprepared by said ball rolling consisting of inner oxidic cores andsurrounding shells of said lead sulphide material on a sintering gratein the form of a charge layer permeable to well gas, igniting saidcharge on the surface, causing oxygen containing gas to pass throughsaid charge so as to effect the sintering without allowing easilyfusible products to flow down through the charge by controlling thecontents of heat consuming and heat liberating components of theindividual balls and cooling the charge rapidly subsequent to saidsintering.

7. A method as claimed in claim 6, characterized by at signs offormation of easily fusible products in the charge decreasing the shellthickness of the balls fed to the sintering grate.

8. In a method for agglomerating and sintering a fine granular leadsulphide material containing at least about 55% lead, whilesimultaneously desulphurizing the material and forming an oxidecontaining product, the improvement which comprises coating particles oflead oxide containing material derived from a sintering operationcarried out previously and having an average particle size within therange of about 5-30 mm. with a thin layer of heat consuming material andsubsequently with a thin layer of said lead sulphide material, both ofsaid shells being applied by ball rolling, the last ball rolling beingcontrolled so as to form balls having a sulphide shell of said leadsulphide material and a thickness within the range of about 1-3 mm.,said lead oxide material being present in an amount of at least about30% and not more than about 65%, applying the material prepared by saidball rolling, consisting of inner oxidic cores and surrounding shells ofsaid lead sulphide material on a sintering grate in the form of a chargelayer permeable to well gas, igniting said charge on the surface,causing oxygen containing gas to pass through said charge withoutallowing easily fusible products to flow down through the charge andcooling said charge rapidly subsequent to said sintering.

9. A method as claimed in claim 8, characterized in that the heatconsuming material is selected from the group which consists ofcarbonates, crystal water containing salts and hydrates.

10. In a method for agglomerating and sintering a fine granular leadsulphide material containing at least about lead, while simultaneouslydesulphurizing the material and forming an oxide containing product, theimprovement which comprises coating particles of lead oxide containingmaterial derived from a sintering operation carried out previously andhaving an average particle size within the range of about 5-30 mm. witha thin layer of oxygen containing lead compounds such as obtained inmetallurgical plants, in form of dusts, fines from crushing operations,slimes and skimmings, as well as a thin layer of said lead sulphidematerial by ball rolling, said ball rolling being controlled so as toform balls having a shell of said lead sulphide material of a thicknesswithin the range of about 1-3 mm., said oxidic material being present inan amount of at least about 30% and not more than about applying thematerial prepared by said ball rolling consisting of inner oxidic coresand shells and surrounding shell of said lead sulphide material on asintering grate in the form of a charge layer permeable to well gas,igniting said charge, causing oxygen contain ing gas to pass throughsaid charge so as to efiect the sintering without allowing easilyfusible products to flow down through the charge and cooling said chargerapidly subsequent to said sintering.

11. A method as claimed in claim 10, characterized by applying a shellof heat consuming material between said layer of oxygen containing leadcompounds and said layer of lead sulphide material.

No references cited.

1. IN A METHOD OF AGGLOMERATING AND SINTERING A FINE GRANULAR LEADSULPHIDE MATERIAL CONTAINING AT LEAST ABOUT 55%LEAD, WHILESIMULTANEOUSLY DESULPHURIZING THE MATERIAL AND FORMING AN OXIDECONTAINING PRODUCT, THE IMPROVEMENT WHICH COMPRISES COATING PARTICLES OFLEAD OXIDE CONTAINING MATERIAL DERIVED FROM A SINTERING OPERATIONCARRIED OUT PREVIOUSLY AND HAVING AN AVERAGE PARTICLE SIZE WITHIN THERANGE OF ABOUT 5-30 MM. WITH A THIN LAYER OF SAID LEAD SULPHIDE MATERIALBY BALL ROLLING, SAID BALL ROLLING BEING CONTROLLED SO AS TO FORM BALLSHAVING A SHELL OF SAID LEAD SULPHIDE MATERIAL OF A THICKNESS WITHIN THERANGE OF ABOUT 1-3 MM. SAID OXIDIC MATERIAL BEING PRESENT IN AN AMOUNTOF AT LEAST ABOUT 30% AND NOT MORE THAN ABOUT 65%, APPLYING THE MATERIALPREPARED BY SAID BALL ROLLING CONSISTING OF INNER OXIDIC CORES ANDSURROUNDING SHELLS OF SAID LEAD SULPHIDE MATERIAL ON A SINTERING GRATEIN THE FORM OF A CHARGE LAYER PERMEABLE TO WELL GAS, IGNITING SAIDCHARGE, CAUSING OXYGEN CONTAINING GAS TO PASS THROUGH SAID CHARGE SO ASTO EFFECT THE SINTERING WITHOUT ALLOWING EASILY FUSIBLE PRODUCTS TO FLOWDOWN THROUGH THE CHARGE AND COOLING SAID CHARGE RAPIDLY SUBSEQUENT TOSAID SINTERING.